In radio frequency (RF) communication systems, an audio message is modulated onto a carrier signal to provide a communication signal which is transmitted over air by means of an RF transmitter. A radio receiver receives the communication signal and demodulates it to recover the audio message which is presented to a speaker through an audio circuit.
Generally, when the received communication signal (or carrier signal) has little or no strength, the output of the demodulator comprises substantial noise in the 10-20 KHz range which is detected and mutes the audio path to the speaker. Squelching the radio prevents undesired noise from being heard, which also decreases power consumption of the receiver because no current is drawn by the speaker.
In the radio receivers, a noise squelch circuit generally provides a squelch signal which disables the audio circuit based on the noise power at the demodulator output. The received signal strength level which unsquelches a radio is commonly referred to as squelch sensitivity of the receiver. The squelch sensitivity of the receiver varies based on the noise characteristics of the demodulator output. Usually, the squelch sensitivity of the receiver is set to different levels by varying an attenuator which sets a threshold for unsquelching the radio.
Conventionally, the noise at the demodulator output after being frequency shaped and rectified is compared to the threshold to determine whether the squelch signal should be generated or not. However, because the noise characteristics of the demodulator output are not stable and may vary under certain conditions, the conventional squelch circuits may squelch or unsquelch the radio prematurely. The noise characteristic variations may depend on such factors as the channel spacing of the receiver (e.g. 25 KHz, 12.5 KHz), the received signal strength level, signal-to-noise ratio of the received signal (i.e. SINAD), and the deviation of the received communication signal (especially in frequency modulation (FM) communication systems). The noise characteristic variations could degrade radio receiver operation unless receiver hardware is modified. Therefore, numerous receiver circuit hardware may be required for accommodating the various operating circumstances. Accordingly, it is desired to improve squelch response of the radio receiver without extensive hardware modifications.